We have synthesized malonaldehyde uniformly labeled with 14C. In the course of this work, we discovered that the common procedure for the preparation of malonaldehyde (i.e. hydrolysis of tetraalkoxypropanes) produces appreciable concentrations of side products. We have determined that two side products of the reaction are fifteen to twenty times more mutagenic than pure malonaldehyde. These compounds would not be formed from malonaldehyde in vivo, so their presence in malonaldehyde preparations will lead to anomalously high estimations of the mutagenicity and carcinogenicity of malonaldehyde. Highly purified malonaldehyde is weakly mutagenic in Salmonella typhimurium strain His D 3052 and it may be carcinogenic as well. Since it is widely produced in animal tissue as a product of prostaglandin H2 metabolism and lipid peroxidation, it may be a mediator of "spontaneous" carcinogenesis. We are, therefore, developing methods for the analysis of malonaldehyde in biological fluids and for the determination of the products of its metabolism in vivo. These experiments and others will serve as a basis for our long term study of the physiological role of malonaldehyde and its involvement in the evolution of pathological states.